The invention relates to a method and apparatus for actively measuring the small signal gain of an amplifier. That is, the invention relates to a method and apparatus for periodically measuring the small signal gain of an amplifier while the amplifier is in operation. The invention also relates to a method and apparatus for actively compensating for differences in the small signal gain among two or more amplifiers.
In recent years, much work has been performed on the development of solid state imaging devices. These devices have been proposed in many forms, depending upon the requirements of the particular application involved. In the field of solid state infrared imaging devices, a device has been proposed and built in which an X-Y addressable array of junction field effect transistors (JFET's) is provided with a pyroelectric target material. This device is disclosed in an article entitled "Solid-state pyroelectric imaging system" by A. Carlson et al. (Proceedings of SPIE, Vol. 267 Staring Infrared Focal Plane Technology, 1981).
The use of an array of JFET's is advantageous, among other reasons, because it greatly reduces amplifier noise. This is achieved because JFET's are inherently low-noise devices, and because an array of JFET's can provide individual amplification of each pyroelectric channel before multiplexing. However, in the use of a JFET array, other problems arise. These problems are the nonuniformities in the characteristics of the individual JFET's (or other amplifiers) in the array. There are two kinds of nonuniformities, namely offset and gain. While offset nonuniformities are readily corrected in the known device, there is no method suggested in this article for correcting gain nonuniformities.